1. Field of Invention
This invention relates generally to pad printing, and in particular to transferring multi-color images.
2. Prior-Art—Pad Printing—FIGS. 1A-1E
Image transfer through pad printing is an old and well-established art. It is a type of offset printing that is used to apply markings and images to a variety of flat, curved, and irregular surfaces, including watch faces, golf balls, and bottles. The steps in prior-art pad printing are shown in cross-section in FIGS. 1A through 1E.
FIG. 1A shows a cross-sectional view of a prior-art pad 140 suspended over cliché 110 after cliché 110 has been inked. A cliché is a stereotype or printing plate which is etched in a pattern to be printed and which is inked. Ink 100 is doctored into image-shaped depressions 120 in cliché 110, in well-known fashion. The flat surface 130 of cliché 110 is scraped clean during the process of doctoring ink 100 into depressions 120. Pad 140 later lifts the ink from the cliché and applies it to the final receiving surface (not shown).
FIG. 1B shows a cross-sectional view of pad 140 in contact with surface 130 of cliché 110 and ink 120. A soft, typically conical, flexible rubber pad 140 is affixed to a rigid plate 145. Plate 145 is rigidly attached to shaft 148. Shaft 148 is connected to a ram (not shown) which moves shaft 148, plate 145, and pad 140 up and down. To pick up ink 100, pad 140 is pressed against cliché 110 with adequate force, between 0.2 and 50 kilograms, to deform pad 140, fully spanning all inked areas. The surface of pad 140 is thereby wetted with ink 100 in the pattern of the image to be printed.
FIG. 1C shows a cross-sectional view of pad 140 with ink 100 which has been removed from cliché 110. Pad 140 is next lifted away from cliché 110. Ink 100 adheres preferentially to pad 140, and is removed from depressions 120.
FIG. 1D shows a cross-sectional view of pad 140, ink 100, and image-receiving surface 150. Pad 140 is next forcibly pressed against a new receiving surface 150 which may be a flat, curved, or irregular object. During this pressing, pad 140 is deformed to the same extent as shown in FIG. 1B. By deforming pad 140 to the same extent as shown in FIG. 1B, an ink pattern identical to the original image-wise pattern of ink-containing depressions 120 is recreated on surface 150.
FIG. 1E shows the final printing step in which pad 140 is removed from surface 150, leaving ink 100 behind. Pad 140 is next lifted away from receiving surface 150. As pad 140 is lifted away, ink 100 leaves the surface of pad 140 and adheres to receiving surface 150. An ink replica of the original image in cliché 110 is thus transferred from cliché 110 to receiving surface 150.
Pad 140 is typically between 5 mm and 20 cm in diameter, and between 5 mm and 10 cm in height. Depressions 120 can be any shape and are typically between 0.25 mm and 5 mm in extent, and 0.025 mm deep.
Although a flat receiving surface 150 is shown, other shapes are possible. Images can as easily be transferred to curved and irregular surfaces.
The preparation of cliché 110 involves well-known photographic processing and etching or dissolving away of the pattern of depressions 120. Each cliché is used to print one color at a time.
Printing of multi-color images requires preparation of more than one cliché. An image composed of two different component colors requires the preparation of two clichés. Three-color images require the preparation of three clichés, and so forth. Full-color, process printing involves the preparation of three or four clichés, one each for cyan, magenta, yellow, and optionally, black. Separation, exposing, and etching of the clichés is time-consuming and expensive.
Printing of multi-color images further requires precise registration of the printed images. This means that the pad must be precisely positioned over the first inked cliché, then again precisely positioned over the receiving surface. Then, the pad must be precisely positioned over the second inked cliché, and again precisely positioned over the receiving surface, and so on. The precision required to obtain visually acceptable images places very stringent requirements on the skill of the operator in preparing the clichés, and in operation and tolerances of the equipment which transfers the ink from the plates to the final receiving surface. These operations require significant expenditures of time and labor.
When any of the steps above contain positional and other errors, these errors can result in improper registration and alignment of the multiple colors and hence failed prints which must be discarded. This results in extra costs to the manufacturer and wasted materials, time, and money. In addition, making one or more clichés for each print costs time and money.